1. Field of the Invention
The present invention relates to an image processing apparatus that superimposes additional information onto image information, a method thereof and a medium storing a program thereof.
2. Description of the Related Art
Research has been actively conducted recently on superimposing special information on image information (image data), for the purpose of, for example, preventing unauthorized copying and tampering with data. Generally, such a technique is known as digital watermarking (superimposing). For example, additional information regarding the creator, presence or absence of use permission, and the like is superimposed on the image information of digitized photographs and paintings. In a recently-standardized technique, additional information is superimposed on the original image information so that the additional information is difficult to distinguish visually, and the image information is distributed via a network such as the Internet. However, it is known that image quality may be degraded when such superimposing processing is performed on image information, and various methods for ameliorating such degradation of image quality have been proposed.
Japanese Patent Laid-Open No. 2004-140764 (U.S. Pat. No. 7,277,557) describes a method for creating a pattern at the time of superimposing according to the size, embedding intensity, and pattern attenuation rate. Particularly, this document describes setting the pattern attenuation rate, which makes it possible to make pattern recognition processing efficient and suppress degradation of the image quality of an input image without generating edges between patterns. Furthermore, Japanese Patent Laid-Open No. 2003-078756 (U.S. Pat. No. 7,164,778) describes a method for changing the intensity of the watermark pattern according to the regional complexity of the image. The document also describes that regional complexity of an image using a comparatively large value in a texture region and a comparatively small value in a flat region or an edge portion can be created, which comparatively improves the robustness of digital watermarks.
With recent advancements in digital watermark techniques, it is becoming possible to superimpose larger amounts of information on images of the same size than used in the past. Under such circumstances, for example, when the amount of information to be added is small, in many cases, a technique for performing superimposing processing on parts of an image (regional superimposing) is becoming more common. However, a new problem has emerged, which is the degradation of image quality caused when the regional superimposing is performed in the proximity of boundaries (the proximity of superimposing boundaries) between a region on which the superimposing processing has been performed and a region on which the superimposing processing has not been performed.
The method described in Japanese Patent Laid-Open No. 2004-140764 aims simply to moderate the interference between patterns; however, the degradation of image quality in the proximity of the superimposing boundaries cannot be decreased effectively. Also, in the method described in Japanese Patent Laid-Open No. 2003-078756, superimposing control is performed according to the characteristics of the image, but no control is performed in consideration of the proximity of the superimposing boundaries when the regional superimposing is performed. Therefore, degradation of the image quality in the proximity of the superimposing boundaries cannot be decreased effectively.
The superimposing processing may be performed on the whole area to avoid degradation of image quality in the proximity of the superimposing boundaries. However, in such a case, when the region that requires superimposing is small, the degradation of image quality is caused even in unnecessary regions. Furthermore, performing the superimposing processing on unnecessary regions prolongs the processing time. Finally, when attempting to avoid degradation of image quality in the proximity of the superimposing boundaries by performing the superimposing processing with an intensity that results in less visual influence, there is the chance that it will be impossible to extract data when that data is to be extracted from the image.